Book - Comparative Embryology of the Vertebrates 1: Difference between revisions

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=Part I The Period of Preparation=
=Part I The Period of Preparation=
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# Luteinizing hormone, LH (ICSH), and  
# Luteinizing hormone, LH (ICSH), and  
# Luteotrophin, LTH.  
# Luteotrophin, LTH.  
==The Testis and Its Relation to Reproduction==
==The Testis and Its Relation to Reproduction==
 
[[Book - Comparative Embryology of the Vertebrates 1-1|'''1. The Testis and Its Relation to Reproduction''']]
 
 


A. Introduction  
A. Introduction  
# General description of the male reproductive system
# Importance of the testis


1. General description of the male reproductive system
2. Importance of the testis


B. Anatomical features of the male reproductive system  
B. Anatomical features of the male reproductive system  
# Anatomical location of the testis
# Possible factors involved in testis descent
# General structure of the scrotum and the testis in mammals  a. Structure of the scrotum  b. General structure of the testis
# Specific structures of the mammalian testis which produce the reproductive cells and the male sex hormone a. Seminiferous tubules b. Interstitial tissue
# The testis of vertebrates in general
# Accessory reproductive structures of the male a. The reproductive duct in forms utilizing external fertilization b. The reproductive duct in species practicing internal fertilization c. Specific activities of the various parts of the male reproductive system


1. Anatomical location of the testis
==The Vertetrate Ovary and Its Relationship to Reproduction==
[[Book - Comparative Embryology of the Vertebrates 1-2|'''2. The Vertebrate Ovary and Its Relation to Reproduction''']]


2. Possible factors involved in testis descent
A. The ovary and its importance


3. General structure of the scrotum and the testis in mammals
B. Preformationism, past and present


a. Structure of the scrotum
C. General structure of the reproductive system of the vertebrate female


b. General structure of the testis
1. General structure of the ovary


4. Specific structures of the mammalian testis which produce the reproductive cells
2. General structure of the accessory reproductive organs
and the male sex hormone


a. Seminiferous tubules
D. Dependency of the female reproductive system on general body conditions


b. Interstitial tissue
1. Inanition


5. The testis of vertebrates in general
2. Vitamins


6. Accessory reproductive structures of the male
a. Vitamin A


a. The reproductive duct in forms utilizing external fertilization
b. Vitamin B


b. The reproductive duct in species practicing internal fertilization
c. Vitamin C


C. Specific activities of the various parts of the male reproductive system
d. Vitamin E


1. Introduction
3. The hypophysis (pituitary gland)


a. Three general functions of the male reproductive system
E. Activities of the ovary in producing the reproductive state


b. Some definitions
1. The ovary as a “storehouse” of oogonia


2. Activities of the testis
2. Position occupied by the primitive female germ cells in the ovarian cortex


a. Seasonal and non-seasonal types of testicular activity
3. Primary, secondary, and tertiary follicles of de Graaf


b. Testicular tissue concerned with male sex-hormone production
4. Hormonal factors concerned with the development of egg follicles


c. Testicular control of body structure and function by the male sex hormone
a. Effects produced by the gonadotrophic hormones of the development of the  
mammalian egg follicle


1) Sources of the male sex hormone
b. Stimulating effects of the gonadotrophins on the ovaries of other vertebrates


2) Biological effects of the male sex hormone
5. Structure of the vertebrate, mature egg follicle


a) Effects upon the accessory reproductive structures
a. Structure of the mature follicle in metatherian and eutherian mammals


b) Effects upon secondary sex characteristics and behavior of the individual
b. Structure of the prototherian egg follicle


c) Effects upon the seminiferous tubules
c. Egg follicles of other vertebrates


d. Seminiferous-tubule activity and formation of sperm
6. Ovulatory process; possible factors controlling ovulation


e. The seminiferous tubule as a sperm-storing structure
a. Process of ovulation in higher mammals


3. Role of the reproductive duct in sperm formation
1) Changing tissue conditions culminating in egg discharge from the ovary


a. Vertebrates without a highly tortuous epididymal portion of the reproductive
2) Hormonal control of the ovulatory process
duct


b. The epididymis as a sperm-ripening structure
b. Ovulation in vertebrate groups other than the higher mammals


c. The epididymis and vas deferens as sperm-storage organs
1) Hen


d. Two types of vertebrate testes relative to sperm formation
2) Frog


4. Function of the seminal vesicles (vesicular glands)
3) Hormonal control of ovulation in lower vertebrates


5. Function of the prostate gland
c. Comparison of the immediate factors affecting egg discharge in the vertebrate
group


6. Bulbourethral (Cowper’s) glands
7. Internal conditions of the ovary as an ovulatory factor


7. Functions of seminal fluid
8. Number of eggs produced by different vertebrate ovaries


a. Amount of seminal fluid discharged and its general functions
9. Spontaneous and dependent ovulation in the mammals and in other vertebrates


b. Coagulation of the semen
10. Egg viability after discharge from the ovary


c. Hyaluronidasc
11. History of the egg follicle after ovulation


d. Accessory sperm
a. Follicles which do not develop a post-ovulatory body


e. Fructose
b. Follicles which develop a post-ovulatory body; formation of the corpus luteum


f. Enzyme-protecting substances
12. Hormones of the ovary and their activities in effecting the reproductive condition


D. Internal and external factors influencing activities of the testis
a. Estrogenic hormone


1. Internal factors
1) Definition and source of production


a. Temperature and anatomical position of the testis
2) The ovary as the normal source of estrogen in the non-pregnant female


b. Body nourishment in relation to testicular function
3) Pituitary control of estrogen formation


c. The hypophysis and its relation to testicular function
4) Effect of estrogen upon the female mammal


2. External environmental factors and testis function
5) Effects of estrogen in other vertebrates


a. Light as a factor
b. Progesterone — the hormone of the corpus luteum


b. Temperature influences
1) Production of progesterone


E. Internal factors which may control seasonal and continuous types of testicular
2) Effects of progesterone
function


F. Characteristics of the male reproductive cycle and its relation to reproductive conditions in the female  
F. Reproductive state and its relation to the reproductive cycles in female vertebrates


1. Sexual cycle in the female mammal


===C. Specific Activities of the Various Parts of the Male Reproductive System===
a. Characteristics and phases of the reproductive cycle


1. Introduction
b. Relation of cstrus and ovulation in some common mammals


a. Three General Functions of the Male Reproductive System
1) Spontaneously ovulating forms (Sexual receptivity of male occurs at or
near time of ovulation)


The activities of the testes and the accessory parts of the male reproductive
2) Dependent ovulatory forms (Sexual receptivity [heat] occurs previous to
system result in the performance of three general functions as follows:
time of ovulation)


# formation of the semen,
c. Non-ovulatory (anovulatory) sexual cycles
# delivery of the semen to the proper place where the sperm may be utilized in the process of fertilization, and
# elaboration of the male sex hormone.  


d. Control of the estrous cycle in the female mammal


b. Some Definitions
e. Reproductive cycle in lower vertebrate females


Semen or seminal fluid is the all-important substance which the male contributes during the reproductive event. It is the product of the entire reproductive system, including special glands of the accessory reproductive structures. The semen is composed of two parts:
G. Role of the ovary in gestation (pregnancy)


(1) The sperm (spermatozoa, spermia) are the formed elements which
1. Control of implantation and the maintenance of pregnancy in mammals
take part in the actual process of fertilization.


(2) The seminal plasma, a fluid part, is a lymph-like substance containing various substances dissolved or mixed in it. These contained substances are important as a protection for the sperm and as an aid to
2. Gestation periods, in days, of some common mammals
the process of fertilization.


With regard to the second function of the male genital system, namely,
3. Maintenance of pregnancy in reptiles and other vertebrates
the delivery of sperm to the site of fertilization, it should be observed that


H. Role of the ovary in parturition or birth of the young


I. Importance of the ovary in mammary-gland development and lactation


Fig. 10. Spermatophores of common urodeles. (Redrawn from Noble: Biology of the  
J. Other possible developmental functions produced by the ovary
Amphibia, New York, McGraw-Hill.) (A) Triturus viridescens. (After Smith.) (B)
Desmognathus fuscus, (After Noble and Weber.) (C) Eiirycea hislineata.


K. Determinative tests for pregnancy


in some vertebrates this is a more simple problem than in others. In those
==The Development of the Gametes or Sex Cells==
forms which practice external fertilization, the male system simply discharges
[[Book - Comparative Embryology of the Vertebrates 1-3|'''3. The Development of the Gametes or Sex Cells''']]
the seminal fluid into the surrounding external medium. However, in those
vertebrates where internal fertilization is the rule, the female system assumes
some of the burden in the transport of the semen to the region where fertilization is consummated, thus complicating the procedure. In these instances,
the male genital tract is called upon to produce added substances to the seminal
fluid which aid in protecting the sperm en route to the fertilization site.


The elaboration of the androgenic or male sex hormone is a most important function. Androgenic or male sex hormone substances are those organic
A. General considerations
compounds which induce maleness, for they aid the development of the male
secondary sex characteristics, enhance the growth and functional development
of the male accessory reproductive structures, and stimulate certain aspects
of spermatogenesis.' Like the estrogens, androgens are not confined to a
particular sex; they have been extracted from the urine of women and other
female animals. The androgens derived from urinary concentrates are androsterone and dehydroisoandrosterone. These two androgens are not as powerful
as that prepared from testicular tissue. Testicular androgen was first isolated
from testicular tissue in 1935 and was given the name testosterone. It also
has been synthesized from cholesterol. It is the most powerful of the androgens
and probably similar, if not identical, with the substance produced in the
testis (Koch, ’42).  


B. Controversy regarding germ-cell origin


2. Activities of the Testis
C. Maturation (differentiation) of the gametes


a. Seasonal and N on-seasonal Types of Testicular Activity
1. General considerations


The testis has two main functions: the production of sperm and formation
2. Basic structure of the definitive sex cell as it starts to mature or differentiate into the male meiocyte (i.e., the spermatocyte) or the female meiocyte (i.e., the oocyte)
of the male sex hormone. In many vertebrates these two activities represent
a continuous procedure during the reproductive life of the male animal. This


3. Nuclear maturation of the gametes


a. General description of chromatin behavior during somatic and meiotic mitoses


Fig. II. Seasonal spermatogenesis and accessory gland development in the ground
b. Reductional and equational meiotic divisions and the phenomenon of crossing over
squirrel, Citellus tridecemlineatus. Stippling below base line shows period of hibernation,
whereas crosshatching reveals the reproductive period. (From Turner: General Endocrinology, Philadelphia, Saunders, after L, J. Wells.)


condition is found in certain tropical fish, in the common fowl and various
c. Stages of chromatin behavior during the meiotic prophase in greater detail
wild tropical birds, and in many mammals, such as man, the dog, bull, stallion,
cat, etc. On the other hand, in the majority of vertebrates these activities of  
the testis are a seasonal affair. This condition is found in most fish, practically
all amphibia, all temperate -zone-inhabiting reptiles, most birds, and many
mammals. Among the latter, for example, are the ferret, deer, elk, fox, wolf,
and many rodents, such as the midwestern ground squirrel. Seasonal periodicity
is true also of the common goose and turkey.


Sperm-producing periodicity is not correlated with any particular season,
1) Leptotene (leptonema) stage
nor is spermatogenesis always synchronized with the mating urge, which in
turn is dependent upon the male sex hormone. In some forms, these two
testicular functions may actually occur at different seasons of the year, as
for example, in the three-spined stickleback, Gasterosteus aculeatus (fig. 15).
(See Craig-Bennett, ’31.) In general, it may be stated that sperm are produced
during the weeks or months which precede the development of the mating
instinct. Many species follow this rule. For example, in the bat of the genus
Myotis, sperm are produced during the late spring and summer months, while
mating or copulation takes place during the fall or possibly early the next
spring (Guthrie, ’33). In the common newt, Triturus viridescens, spermatogenesis comes to pass during the warm months of the summer, and sperm
are discharged from the testis into the reproductive ducts during the late fall
and early spring, while copulation is accomplished in the early spring. The
testes in this species are quiescent during the cool winter months. In the
midwestern ground squirrel, Citellus tridecemlineatus, spermatogenesis begins
in November and is marked during February and March (fig. 11). The animal
hibernates away the winter months and emerges the first part of April in a
breeding condition. Mating occurs in the early spring (Wells, ’35). In the
garter snake, Tharnnophis radix, sperm are produced in the testes in the summer months, stored in the epididymides during the hibernation period in the
fall and winter, and used for copulation purposes in the spring (Cieslak,
’45). Again, in the Virginia deer, Odocoileus virginianus borealis, studied by
Wislocki (’43), active spermatogenesis is realized during the summer and
early autumn months, while the mating season or “rut” which results from
the driving power of the male sex hormone, is at its peak in October and
November (fig. 12). In the fox. Bishop (’42) observed spermatogenesis to
begin in the late fall months, while mating is an event of the late winter and
early spring. In April and May the seminiferous tubules again assume an
inactive state (fig. 13). In the common frog, Rana pipiens, spermatogenesis
is present in the summer months and morphogenesis of spermatids into sperm
happens in large numbers during September, October, and November. Sperm
are stored in the testis over the winter, and the mating instinct is awakened
in the early spring (Glass and Rugh, ’44). Following the mating season in
spring and early summer the testis of the teleost, Fundulus heteroclitus, is
depleted of sperm until the next winter and spring (Matthews, ’38).


As the seasonal type of testicular activity is present in a large number of
2) Zygotene or synaptene (zygonema) stage
vertebrate species, it seems probable that it represents the more primitive or  
fundamental type of testicular functioning.


b. Testicular Tissue Concerned with Male Sex-hormone Production
3) Pachytene (pachynema) stage


While one cannot rule out the indirect effects which activities of the seminiferous tubules may have upon the functioning of the testis as a whole, including the interstitial tissue, direct experimental evidence and other observations suggest that the interstitial tissue holds the main responsibility for
4) Diplotene (diplonema) stage
the secretion of the male sex hormone, testosterone, or a substance very
closely allied to it. For example, if a testis from an animal possessing a permanent scrotum is removed from the inguinal bursa and placed within the
peritoneal cavity, the seminiferous tubules tend to degenerate, but the interstitial tissue remains. The sex hormone, under these circumstances, continues
to be produced. Again, males having cryptorchid testis (i.e., testes which have
failed in their passage to the scrotum) possess the secondary sex characteristics of normal males but fail to produce sperm cells. Also, it has been demonstrated that the mammalian fetal testis contains the male sex hormone. However, in this fetal condition, the seminiferous tubules are present only in an
undeveloped state, whereas interstitial tissue is well differentiated. It is probable
in this case that the interstitial tissue of the fetal testis responds to the
luteinizing hormone in the maternal blood.


5) Diakinesis


d. Peculiarities of nuclear behavior in the oocyte during meiosis; the germinal vesicle


Fig. 12. Sections of the testis of the deer, Odocoileus virginianus borealis. (After
e. Character of the meiotic (maturation) divisions in the spermatocyte compared with those of the oocyte
Wislocki.) (A) Seminiferous tubules of deer in June. Observe repressed state of tubules
and absence of sperm. (B) Epididymal duct of same deer. Observe absence of sperm
and smaller diameter of duct compared with (D). (C) Seminiferous tubules of October deer; spermatogenic activity is marked. (D) Epididymal duct, showing well-developed
epididymal tube and presence of many sperm.


1) Dependent nature of the maturation divisions in the female meiocyte


2) Inequality of cytoplasmic division in the oocyte


In hypophysectomized male rats injected with dosages of pure folliclestimulating hormone (FSH) or with small doses of pure luteinizing hormone (LH; ICSH), the seminiferous tubules of the testis respond and spermatogenesis occurs. However, the interstitial tissue remains relatively unstimulated and the accessory structures continue in the atrophic state. If larger
f. Resume of the significance of the meiotic phenomena
doses of the luteinizing factor are given, the interstitial tissue responds and
the secondary sexual characters are developed, showing a relationship between
interstitial activity and sex-hormone production. (Consult Evans and Simpson
in Pincus and Thimann, ’50, pp. 355, 356.)


From certain species whose reproductive activities are confined to a particular season of the year, there also comes evidence that the interstitial tissue is the site of sex-hormone production. In the behavior of testicular tissue in
4. Cytosomal (Cytoplasmic) maturation of the gametes
the stickleback, Gasterosteus, as shown by van Oordt (’23) and Craig-Bennett
(’31) sperm are produced actively in the seminiferous tubules during one
period of the year when the interstitial tissue is in an undeveloped condition.
The secondary sex characters also are in abeyance at this season of the year.  
However, during the months immediately following sperm production, sperm
are stored within the seminiferous tubules and active spermatogenesis is absent.
When the seminiferous tubules thus have completed their spermatogenic activity, the interstitial tissue begins to increase, followed by a development of
secondary sex characteristics (figs. 14, 15). A similar difference in the rhythm
of development of these two testicular tissues can be shown for many other
vertebrates. All of these suggestive facts thus serve to place the responsibility
for male sex-hormone production upon the interstitial tissue, probably the
cells of Leydig.


a. General aspects of the cytoplasmic maturation of the gametes


b. Morphogenesis (spermiogenesis) ( sperm ioteleosis) of the sperm


Fig. 13. Sections of seminiferous tubules of silver fox. (After Bishop.) (A) Regressed state of tubules following breeding season. (B) Tubule from fox during the breeding season, characterized by active spermatogenesis.
1) Types of sperm


2) Structure of a flagellate sperm


a) Head


Fig. 14. Sections of the testis of the stickleback (Gasterosteus pungitius). (Modified
b) Neck
from Moore, ’39, after Van Oordt.) Cf. fig. 13, (A) Spermatogenic activity with many


formed sperm in seminiferous tubules before the mating season, interstitial tissue in
c) Connecting body or middle piece
abeyance. (B) At mating period. Interstitial tissue well developed, spermatozoa stored
in the tubules with spermatogenic activity absent.


d) Flagellum


3) Spermiogenesis or the differentiation of the spermatid into the morphologically differentiated sperm


Fig. 15. Seasonal reproductive cycle in the stickleback {Gasterosteus aculeatus). Cf.
a) Golgi substance and acroblast; formation of the acrosome
fig. 14. Breeding season is indicated by crosshatching below base line. Observe that
spermatogenic activity follows rise of temperature, whereas interstitial-tissue and sexcharacter development occur during ascending period of light. (Redrawn from Turner:
General Endocrinology, Philadelphia, Saunders, modified from Craig-Bennett, 1931.)


b) Formation of the post-nuclear cap


c. Testicular Control of Body Structure and Function by the Male
c) Formation of the proximal and distal centrioles; axial filament
Sex Hormone


d) Mitochondrial material and formation of the middle piece of the sperm


1) Sources of the Male Sex Hormone. Testosterone is prepared from testicular extracts. It is the most potent of the androgens and is believed to be
e) The cytoplasm, axial filament, mitochondria, and tail formation
the hormone produced by the testis. The chemical formula of testosterone is:


on


c. Cytoplasmic differentiation of the egg


ciu
1) Types of chordate eggs


a) Homolecithal (isolecithal) eggs


o
b) Telolecithal eggs


2) Formation of the deutoplasm


CHs
3) Invisible morphogenetic organization within the cytoplasm of the egg


J\J\J
4) Polarity of the egg and its relation to body organization and bilateral symmetry of the mature egg


The testis, however, is not the only site of androgen formation. As mentioned above, androgens are found in the urine of female animals, castrates,
5) Membranes developed in relation to the oocyte; their possible sources of origin
etc. It seems probable that the suprarenal (adrenal) cortex may secrete a
certain androgenic substance, possibly adrenosterone, a weak androgen. Many
androgens have been synthesized also in the laboratory (Schwenk, ’44).


2) Biological Effects of the Male Sex Hormone. The presence of the male
a) Chorion in St ye la
sex hormone in the male arouses the functional development of the accessory
reproductive structures, the secondary sexual characters, and also stimulates
the development of the seminiferous tubules.


a) Effects upon the Accessory Reproductive Structures. Castration or removal of the testes from an animal possessing a continuous type
b) Egg membranes of Amphioxus
of testicular activity produces shrinkage, and a general tendency toward atrophy, of the entire accessory reproductive structures. Injection of testosterone
or other androgens under such conditions occasions a resurgence of functional development and enlargement of the accessory structures (fig. 16).
Moreover, continued injections of the androgen will maintain the accessories
in this functional state (Moore, ’42; Dorfman in Pincus and Thimann, ’50).
Similarly, under normal conditions in those vertebrates which possess the
seasonal type of testicular function, the accessory reproductive organs shrink
in size with a loss of functional activity when the testis undergoes regression
during the period immediately following the active season. An enlargement
and acquisition of a normal functional condition of the accessories follows
testicular development as the breeding season again approaches (Bishop, ’42;
Wislocki, ’43; Matthews, ’38; Turner, C. L., ’19). (Compare figs. 12A-D.)


b) Effects upon Secondary Sex Characteristics and Behavior of  
c) Vitelline membrane and zona radiata of elasmobranch fishes  
THE Individual. In addition to the primary effects upon the reproductive
system itself, the androgens induce many other secondary structures and alterations of the physiology and behavior of the individual. The influence of the
testicular hormone has been demonstrated in all of the vertebrate groups
from fishes to mammals (Dorfman in Pincus and Thimann, ’50). Examples
of testosterone stimulation are: the singing and plumage of the male bird;
hair development of certain mammals; the crowing and fighting, together
with spur, comb, and wattle growth in the rooster. The disagreeable belligerency and positive energy drive of the bull, stallion, or human male may
be attributed, largely, to the action of testicular hormone. However, lest we
disparage this aggressive demeanor unduly, it should be recognized that upon
such explosive force rests the preservation of species and races in some instances. As an example, witness that hairy dynamo of the barren northern
tundras, the bull muskox, whose fiery pugnaciousness when the need arises
undoubtedly has been a strong factor in the preservation of this species.


An excellent example of the effect of testosterone is shown in the development of antlers and change in behavior of the Virginia deer, Odocoileus virginianus borealis (Wislocki, ’43). In the northern climate, the testes and male
d) Zona radiata of teleost fishes
accessory organs reach a profound condition of regression in April and May.
Growth of the new antlers starts at this time, and during the late summer the
antlers grow rapidly and begin to calcify. During the summer, also, the testes
develop rapidly, and spermatogenesis results. Loss of the “velvet” covering of
the antlers is experienced during September, and mating is the rule in October
and November. The antlers are shed in midwinter. If the testes are removed
after the naked antler condition is reached, the antlers are shed rapidly. Testosterone administered to does or to young males which have been castrated
induces the development of antlers. The general scheme of antler development
suggests, possibly, that the testicular hormone acts upon an anterior pituitary
factor, and this activated factor in turn initiates antler growth. Hardening of
the antlers and loss of velvet results from testosterone stimulation. Loss of
the antler is synchronized with a decrease in the amount of testosterone in
the blood stream, accompanied by the acquisition of a docile, non-belligerent,
more timid behavior.


c) Effects upon the Seminiferous Tubules. Testosterone has a stimulating effect upon the seminiferous tubule and sperm formation. This matter
e) Vitelline membrane (zona radiata) in amphibia
is discussed in Chap. 3.


d. Seminiferous-tubule Activity and Formation of Sperm
f) Zona radiata (zona pellucida) of the reptile oocyte
See Chap. 3.


e. The Seminiferous Tubule as a Sperm-storing Structure
g) Vitelline membrane (zona radiata) of the hen’s egg


See p. 3 1 .
h) Membranes of the mammalian oocyte


3. Role of the Reproductive Duct in Sperm Formation
5. Physiological maturation of the gametes


a. Vertebrates Without a Highly Tortuous Epididyrnal Portion of the  
a. Physiological differentiation of the sperm
Reproductive Duct


In a large number of vertebrates, morphologically developed sperm pass
b. Physiological ripening of the female gamete
from the testis through the efferent ductules of the epididymis (vasa efferentia)
to the epididyrnal duct where they remain for varying periods. However, in
many vertebrates the anterior (proximal) portion of the sperm duct does not
form a tortuous epididyrnal structure similar to that found in other vertebrates. This condition is present in the common frog, Rana; in the hellbender,
Cryptobranchus; in the bowfin, Amia; etc. Because of this fact, the sperm
pass directly into the vas deferens or sperm duct (Wolffian duct) without
undergoing a sojourn through a convoluted epididyrnal portion of the duct.


Correlated with the type of testis and sperm-duct relationship in the frog,
D. Summary of egg and sperm development
is the fact that one may obtain viable, fertilizing sperm directly from the
testis. For example, if one removes the testis from a living frog and macerates
it in pond water or in an appropriate saline solution, active sperm are obtained
which are capable of fertilizing eggs in a normal manner. That is, the frog
testis matures sperm morphologically and physiologically. This type of testicular maturation is characteristic of many of the lower vertebrates possessing
simple reproductive ducts.


 
[[Book - Comparative Embryology of the Vertebrates 1|'''Part I - The Period of Preparation''']]: [[Book - Comparative Embryology of the Vertebrates 1-1|1. The Testis and Its Relation to Reproduction]] | [[Book - Comparative Embryology of the Vertebrates 1-2|2. The Vertebrate Ovary and Its Relation to Reproduction]] | [[Book - Comparative Embryology of the Vertebrates 1-3|3. The Development of the Gametes or Sex Cells]]
 
Fig. 16. Effects of the male sex hormone upon the functional development of the
accessory reproductive structures of the male rat. (After Turner: General Endocrinology ,
Philadelphia, Saunders, p. 324.) (A) Normal male rat condition produced by injection of crystalline male sex hormone for 20 days into castrate before autopsy. (B) Castrated
male litter mate of (A) receiving no replacement therapy.
 
 
Fig. 17. Diagrammatic drawings of the two types of testicular-reproductive relationships occurring in the vertebrate group. (A) Simplified type of reproductive duct connected with the testis by means of efferent ductules. The duct-testis relationship of many
telepst fishes is similar to this but does not possess the efferent ductules, the sinus-like
reproductive duct being attached directly to the testis. Sperm cells (spermatozoa) are
matured and stored within the testis. This type of relationship generally is found where
fertilization is external or where sperm are discharged all at once during a short reproductive period. (B) More complicated variety of reproductive duct, connected with the
testis by means of efferent ducts, but possessing an anterior twisted portion, the epididymal
duct in which the sperm are stored and physiologically matured. This type of duct
generally is found in those vertebrates which utilize internal fertilization and where
sperm are discharged over a short or extended reproductive period.
 
 
 
b. The Epididymis as a Sperm-ripening Structure
 
On the other hand, in those forms which possess an anterior convoluted
epididymal portion of the reproductive duct, the journey of the sperm through
this portion of the duct appears to be necessary in order that fertilizable sperm
may be produced. In mammals it has been shown that the epididymal journey
somehow conditions the physiological ripening of the sperm. Sperm taken from the mammalian testis will not fertilize; those from the caudal portion
of the epididymis will, provided they have been in the epididymis long enough.
Under normal conditions sperm pass through the epididymis slowly, and retain
their viability after many days’ residence in this structure. Sperm prove to be
fertile in the rabbit epididymis up to about the thirty-eighth day; if kept
somewhat longer than this, they become senile and lose the ability to fertilize,
although morphologically they may seem to be normal (Hammond and Asdell,
’26). In the rat, they may live up to 20 to 30 days in the epididymis and still
be capable of fertilization (Moore, ’28). It has been estimated that the epididymal journey in the guinea pig consumes about two weeks, although they
may live and retain their fertilizing power as long as 30 days in epididymides
which have been isolated by constriction (Moore and McGee, ’28; Young,
’31; Young, ’31b). It is said that in the bull, sperm within the epididymis
may live and be motile for two months. As a result of these facts, it may be
concluded that the epididymal journey normally is a slow process, and that
it is beneficial for the development of sperm “ripeness” or ability to fertilize.
 
c. The Epididymis and Vas Deferens as Sperm-storage Organs
 
Along with the maturing faculty, the epididymal duct and vas deferens also
act as sperm-storage organs. As observed on p. 23, in the bat, Myotis, sperm
are formed in great numbers in the seminiferous tubules and pass to the
epididymal duct where they are stored during the fall, winter, and early spring
months; the epididymal journey thus is greatly prolonged in this species. In
the ovoviviparous garter snake, Thamnophis radix, sperm are produced during
the summer months; they pass into the epididymides during early autumn and
are stored there during the fall and winter. In the mammal, sperm are stored
in the epididymal duct.
 
Aside from its main purpose of transporting sperm to the exterior (see
sperm transport, p. 177), the caudal portion of the sperm duct or vas deferens
also is capable of storing sperm for considerable periods of time. In the
common perch, Perea ftavescens, sperm are developed in the testes in the
autumn, pass gradually into the accessory reproductive ducts, and are stored
there for five or six months until the breeding season the following spring
(Turner, C. L., ’19). Again, in mammals, the ampullary region of the vas
deferens appears to be a site for sperm storage. For example, the ampulla of
the bull sometimes is massaged through the rectal wall to obtain sperm for
artificial insemination. In this form sperm may be stored in the ampulla and
still be viable, for as long as three days. Similarly, in lower vertebrates large
numbers of sperm may be found in the posterior extremities of the vas deferens
during the breeding season. Thus, the reproductive duct (and its epididymal
portion when present) is instrumental in many vertebrate species as a temporary storage place for the sperm.
 
 
d. Two Types of Vertebrate Testes Relative to Sperm Formation
 
The importance of the epididymal duct in many vertebrates and its relative
absence in others, focuses attention upon the fact that in many vertebrate
species sperm are produced, stored, and physiologically matured entirely
within the confines of the testis (frog, bowfin, stickleback, etc.). The reproductive duct under these circumstances is used mainly for sperm transport.
In many other vertebrate species sperm are morphologically formed in
the testis and then are passed on into the accessory structures for storage and
physiological maturation. Functionally, therefore, two types of testes and two
types of accessory reproductive ducts are found among the vertebrate group
of animals (fig. 17). It naturally follows that the testis which produces, stores,
and physiologically matures sperm is best adapted for seasonal activity, particularly where one female is served during the reproductive activities. That
is, it functions as an “all at one time” spawning mechanism. On the other hand,
that testis which produces sperm morphologically and passes them on to a
tortuous epididymal duct for storage and physiological maturing is best adapted
for the continuous type of sperm production or for the service of several
females during a single seasonal period. The sperm, under these conditions,
pass slowly through the epididymal duct, and, therefore, may be discharged
intermittently.
 
4. Function of the Seminal Vesicles (Vesicular Glands)
 
The seminal vesicles show much diversity in their distribution among various mammals. Forms like the cat, dog, opossum, rabbit, sloth, armadillo, whale,
do not possess them, while in man, rat, elephant, mouse, they are welldeveloped structures. It was formerly thought that the seminal vesicles in
mammals acted as a storehouse for the sperm, hence the name. In reality
they are glandular structures which add their contents to the seminal fluid
during the sexual act.
 
5. Function of the Prostate Gland
 
The prostate gland also is a variable structure and is found entirely in the
marsupial and eutherian mammals. In marsupials it is confined to the prostatic portion of the urethral wall; in man it is a rounded, bulbous structure
which surrounds the urethra close to the urinary bladder. In many other
mammals it is a much smaller and less conspicuous structure. It discharges
its contents into the seminal fluid during the orgasm. It is probable that the
prostatic and vesicular fluids form the so-called “vaginal plug’’ in the vagina
of the rat, mouse, etc.
 
6. Bulbourethral (Cowper’s) Glands
 
The bulbourethral glands are absent in the dog but present in most other
mammals. In marsupials and monotremes these structures are exceptionally
well formed. In the opossum there are three pairs of bulbourethral glands.
The mucous contents of these and other small urethral glands are discharged
at the beginning of the sexual climax and, as such, become part of the seminal fluid.
 
 
7. Functions of Seminal Fluid
a. Amount of Seminal Fluid Discharged and Its General Functions
 
As stated previously, the semen or seminal fluid is composed of two parts,
the sperm cells (spermia; spermatozoa) and the seminal plasma. The presence
of the sperm cells represents the most constant feature, although they may
vary considerably from species to species in size, shape, structure, and number
present. The seminal plasma varies greatly as to composition and amount
discharged.
 
The quantity of seminal fluid discharged per ejaculate and the relative
numbers of sperm present in man and a few other vertebrate species associated with him are as follows:*
 
 
Species
 
Volume of Single Ejaculate,
Most Common Value, in CC.
 
Sperm Density in Semen,
Average Value, per CC.
 
Boar
 
250
 
CC.
 
100,000,000
 
per
 
CC.
 
Bull
 
4-5
 
CC.
 
1,000,000,000
 
per
 
CC.
 
Cock
 
0.8
 
CC.
 
3,500,000,000
 
per
 
CC.
 
Dog
 
6
 
CC.
 
200,000,000
 
per
 
CC.
 
Man
 
3.5
 
CC.
 
100,000,000
 
per
 
CC.
 
Rabbit
 
1
 
CC.
 
700,000,000
 
per
 
CC.
 
Ram
 
1
 
CC.
 
3,000,000,000
 
per
 
CC.
 
Stallion
 
70
 
CC.
 
120,000,000
 
per
 
CC.
 
Turkey
 
0.3
 
CC.
 
7,000,000,000
 
per
 
CC.
 
 
* Modified from Mann (’50).
 
 
Two important branches of study involving the semen pertain to:
 
(1) the chemical and physiological nature and numerical presence of the
sperm, and
 
(2) the physiology and biochemistry of the seminal plasma.
 
(See Mann, ’50, for discussion and bibliography.) As a result of the studies
thus far, a considerable body of information has been accumulated.
 
The main function of the semen, including the plasma and accessory sperm,
appears to be to assist the sperm cell whose chance fortune it is to make contact with the egg. Once this association is accomplished, the egg seemingly
takes over the problem of fertilization. The seminal plasma and the accessory
numbers of sperm appear to act as an important protective bodyguard and also as an aid for this event. Modern research emphasizes, therefore, that the
work of the male reproductive system is not complete until this contact is
made.
 
 
b. Coagulation of the Semen
 
In many mammalian species, the semen tends to coagulate after its discharge from the male system. In the mouse, rat, guinea pig, opossum, rhesus
monkey, etc., the semen coagulates into a solid mass, the vaginal plug, once
it reaches the vagina of the female. The probable function of the vaginal
plug is to prevent the semen from seeping out of the vagina. The formation
of this plug may be due to a protein present in the contents of the seminal
vesicle which comes in contact with the enzyme, vesiculase. In the rat and
guinea pig this catalyst probably is produced by the “coagulating gland,” a
specialized structure associated with the seminal vesicles in these forms. Some
of it also may come from the prostate.
 
Coagulation of the seminal fluid also occurs in man, stallion, and boar but
it is entirely absent in the dog, bull, and many other animals. Human semen
coagulates immediately after discharge but liquefies a short time afterward.
This liquefaction may be due to the presence of two enzymes, fibrinogenase
and fibrinolysin, found in human semen and both derived from the prostate.
These enzymes are found also in dog semen. In the latter their property of
inhibiting blood coagulation may be of use where considerable amounts of
blood may be present in the female genital tract at the onset of full estrous
conditions. Another important contribution of the prostate gland is citric acid.
Its role is not clear but it may enter into the above coagulation-liquefaction
process (Mann, ’50, p. 348).
 
 
c. Hyaluronidase
 
Various enzymes have been demonstrated to be present in the semen of
certain invertebrates and vertebrates. One such enzyme is hyaluronidase which
appears to be produced in the testes of the rat, rabbit, boar, bull, and man.
It is not found in the testes of vertebrates below the mammals. Its specific
function is associated with the dispersal of the follicle cells surrounding the
egg; in so doing it may aid the process of fertilization in mammals.
 
d. Accessory Sperm
 
One sperm normally effects a union with the egg in fertilization. Accessory
sperm may enter large-yolked eggs, but only one is intimately involved in
the union with the egg pronucleus. However, what is meant by accessory
sperm here is the large number of sperm which normally clusters around
the egg during the fertilization process in many animal species. A suggestion
of a function for these accessory sperm follows from the fact that hyaluronidase may be extracted from the semen, presumably from the sperm themselves. Rowlands (’44) and also Leonard and Kurzrok (’46) have shown
that a seminal fluid deficient in sperm numbers may fertilize if hyaluronidase
extracted from sperm (?) is added to such a weakened sperm suspension.
The implication is that the accessory sperm thus may act as “cupbearers”
for the one successful sperm in that they carry hyaluronidase which aids in
liquefying the follicle cells and other gelatinous coating material around the egg.
 
e. Fructose
 
An older concept in embryology maintained that sperm were unable to
obtain or utilize nourishment after they departed from the testis. More recent
investigation has shown, however, that sperm do utilize certain sugar materials,
and that their survival depends upon the presence of a simple sugar in the
medium in which they are kept. (See Mann, ’50.)
 
The sugar that is found normally in semen is fructose. It varies in quantity
from species to species, being small in amount in the semen of the boar or
stallion but considerably larger in quantity in the seminal fluid of the bull,
man, and rabbit. The seat of origin of this sugar appears to be the seminal
vesicle, at least in man, although the prostate may also be involved, particularly in the rabbit and also in the dog. The dog, however, has but a small
amount of fructose in the seminal discharge. The real function of seminal
fructose “might be as a readily utilizable store of energy for the survival of
motile spermatozoa” (Mann, ’50, p. 360).
 
f. Enzyme-protecting Substances
 
Runnstrom (personal communication) and his co-workers have demonstrated that the fertilizing life of sea-urchin sperm is increased by certain substances found in the jelly coat of the sea-urchin egg. Presumably these
substances are protein in nature, and, according to Runnstrom, they may
act to preserve the enzyme system of the sperm. Similarly, the seminal fluid
may act to preserve the enzyme system of the sperm, while en route to the
egg, especially within the female genital tract.
 
===D. Internal and External Factors Influencing Activities of the Testis===
 
Conditions which influence testicular activity are many. Many of the factors are unknown. Nevertheless, a few conditions which govern testis function
have been determined, especially in certain mammalian species. The general
results of experimental determination of some of the agents which affect
testicular function are briefly outlined below.
 
1. Internal Factors
 
a. Temperature and Anatomical Position of the Testis
 
It is well known that in those mammals which have a permanent scrotal
residence of the testes failure of the testis or testes to descend properly into the scrotum results in a corresponding failure of the seminiferous tubules
to produce sperm. In these instances the testis may appear shriveled and
shrunken (fig. 18). However, such cryptorchid (ectopic) conditions in most
cases retain the ability to produce the sex hormone at least to some degree.
A question therefore arises relative to the factors which inhibit seminiferous
tubule activity within the cryptorchid testis.
 
The failure of cryptorchid testes to produce viable sperm has been of
interest for a long time. Observations have demonstrated that the more hidden
 
 
 
Fig. 18. Experimental unilateral cryptorchidism in adult rat. The animal's left testis
was confined within the abdominal cavity for six months, whereas the right testis was
pernfitted to reside in the normal scrotal position. Observe the shrunken condition of the
cryptorchid member. (After Turner: General Endocrinology, Philadelphia, Saunders.)
 
 
the testis (i.e., the nearer the peritoneal cavity) the less likely are mature
sperm to be formed. A testis, in the lower inguinal canal or upper scrotal
area is more normal in sperm production than one located in the upper
inguinal canal or inside the inguinal ring. Studies made upon peritoneal and
scrotal temperatures of rats, rabbits, guinea pigs, etc., demonstrate a temperature in the scrotum several degrees lower than that which obtains in
the abdomen. These observations suggest that the higher temperature of the
non-scrotal areas is a definite factor in bringing about seminiferous tubule
injury and failure to produce sperm.
 
With this temperature factor in mind, Dr. Carl R. Moore (in Allen,
Danforth, and Doisy, ’39) and others performed experiments designed to
test its validity as a controlling influence. They found that confinement alone
of an adult guinea pig testicle in the abdomen led to marked disorganization
of all seminiferous tubules in seven days. After several months of such confinement the seminiferous tubules experience marked degenerative changes
and only Sertoli cells remain (fig. 19A, B). The interstitial tissue, however,
is not greatly impaired. If such a testis is kept not too long within the abnormal
position and once again is returned to the scrotum, spermatogenesis is rejuvenated (fig. 20A, B). In a second experiment, the scrotum of a ram was
encased loosely with insulating material; a rapid degeneration of the seminiferous tubules followed. Young (’27, ’29) in a third type of experiment found
that water 6 to 7° warmer than the body temperature applied to the external
aspect of the guinea-pig testis for a 15-minute period evoked degenerative
 
 
 
Fig. 19. Sections of experimental, cryptorchid, guinea-pig, seminiferous tubules and
interstitial tissue. (Modified from C. R. Moore in Sex & Internal Secretions, Williams &
Wilkins, Baltimore, 1939.) (A) Testis confined to abdomen for three months. (B)
 
Testis confined to abdomen for six months. Observe degenerate state of seminiferous
tubule after six months’ confinement. Interstitial tissue not greatly affected by confinement.
 
 
changes with temporary sterility (fig. 21). Recovery, however, is the rule in
the latter instance. Summarizing the effects of such experiments involving
temperature, Moore (in Allen, Danforth, and Doisy, ’39, p. 371) concludes:
“The injury developing from applied heat, although more rapidly effective,
is entirely similar to that induced by the normal body temperature when the
testicle is removed from the scrotum to the abdomen.”
 
The position of the scrotum and its anatomical structure is such as to
enhance its purpose as a regulator of testicular temperature (figs. 2, 6). When
the surrounding temperature is cold, the contraction of the dartos muscle
tissue of the scrotal skin contracts the scrotum as a whole, while the contraction of the cremaster muscle loops pulls the testes and the scrotum closer
to the body, thus conserving the contained heat. When the surrounding temperature is warm, these muscles relax, producing a more pendulous condition
to permit heat loss from the scrotal wall.
 
In accordance with the foregoing description of the scrotum as a necessary
thermoregulator for the testis, it has been further shown for those mammals
which possess a scrotum that testis grafts fare much better when transplanted
to the scrotal wall or into the anterior chamber of the eye (Turner, C. D., '48). The anterior chamber of the eyeball possesses a temperature much
cooler than the internal parts of the body.
 
 
 
 
Fig. 20. Sections of testis during and after abdominal confinement. (Modified from
C. R. Moore in Sex & Internal Secretions, Williams & Wilkins, Baltimore, 1939.) (A)
 
Section of left testis to show degenerate state of seminiferous tubules after 24 days of
abdominal confinement. (B) Section of right testis 74 days after replacement in scrotum.
Observe spermatogenic activity in tubules.
 
 
 
Fig. 21. Effect of higher temperature applied to external surface of guinea-pig testis.
Water, 47®, was applied to surface of scrotum for period of 10 minutes. Testis was
removed from animal 12 days after treatment. Seminiferous tubules are degenerate.
(Modified from Moore, ’39; see also Young, ’27, J. Exp. Zool., 49.)
 
 
 
Two types of seminiferous tubules are thus found in mammals. In a few
mammalian species (see p. 6) the temperature of the peritoneal cavity is
favorable to the well-being of the seminiferous tubule; in most mammalian
species, however, a lower temperature is required. On the other hand, the
activities of the interstitial tissue of the testis appear to be much less sensitive
to the surrounding temperature conditions, and the male sex hormone may
be produced when the testes are removed from the scrotum and placed within
the peritoneal cavity.
 
With regard to the functioning of the testis within the peritoneal cavity
of birds it has been suggested that the air sacs may function to lower the
temperature around the testis (Cowles and Nordstrom, ’46). In the sparrow,
Riley (’37) found that mitotic activity in the testis is greatest during the
early morning hours when the bird is resting and the body temperature is
lower, by 3 or 4° C.
 
b. Body Nourishment in Relation to Testicular Function
 
The testis is a part of, and therefore dependent upon, the well-being of
the body as a whole. However, as observed in the preceding pages the interstitial cells and their activities in the production of the male sex hormone
are less sensitive to the internal environment of the body than are the seminiferous tubules.
 
The separation of these two phases of testicular function is well demonstrated during starvation and general inanition of the body as a whole. A
falling off of sperm production is a definite result of starvation diets, although
the germinative cells do not readily lose their ability to proliferate even after
prolonged periods of starvation. But the interstitial cells and the cells of
Sertoli are not as readily affected by inadequate diets or moderate starvation
periods. Sex drive may be maintained in a starving animal, while his ability
to produce mature, healthy sperm is lost. On the other hand, long periods
of inanition also affect sex hormone production and the sexual interests of
the animal.
 
Aside from the abundance of food in a well-rounded dietary regime, adequate supplies of various vitamins have been shown to be essential. Vitamin
Bi is essential to the maintenance of the seminiferous tubules in pigeons.
Pronounced degenerative changes in the seminiferous tubules of rats and
other mammals occur in the absence of vitamins A and E (Mason, ’39).
Prolonged absence of vitamin E produces an irreparable injury to the testis
of rats; injury produced by vitamin A deficiency is reparable. The B-complex
of vitamins seems to be especially important for the maintenance of the
accessory reproductive structures, such as the prostate, seminal vesicles, etc.
The absence of vitamin C has a general body effect, but does not influence the testis directly. Spme of these effects may be mediated through the pituitary
gland. As vitamin D is intimately associated with the mineral metabolism of
the body, it is not easy to demonstrate its direct importance.
 
c. The Hypophysis and Its Relation to Testicular Function
 
The word “hypophysis” literally means a process extending out below.
The early anatomists regarded the hypophysis cerebri as a process of the
brain more or less vestigial in character. It was long regarded as a structure
through which waste materials from the brain filtered out through supposed
openings into the nasal cavity. These wastes were in the form of mucus or
phlegm, hence the name “pituitary,” derived from a Latin word meaning
“mucus.” The word pituitary is often used synonymously with the word
hypophysis.
 
The hypophysis is made up of the pars anterior or anterior lobe, pars
intermedia or intermediate lobe, and a processus infundibuli or posterior
lobe. The anterior lobe is a structure of great importance to the reproductive
system; its removal (ablation) results in profound atrophic changes throughout the entire reproductive tract.
 
The importance of the pituitary gland in controlling reproductive phenomena was aroused by the work of Crowe, Cushing, and Homans (TO)
and by Aschner (’12) who successfully removed the hypophysis of young
dogs. One of the first fruits of this work was a demonstration of the lack of
genital development when this organ was removed. Since that time many
 
the other cohabitants of man — rats, mice, cats, rabbits, etc. — have been
hypophysectomized, and in all cases a rapid involution and atrophy of the
genital structures results from pituitary removal. The testis undergoes profound shrinkage and regression following hypophysectomy, the degree of
change* varying with the species. In the rooster and monkey, for example,
regressive changes are more marked than in the rat. (Consult Smith, ’39, for
data and references.)
 
A striking demonstration of the influence of the hypophysis upon the
genital tract is the result of its removal from a seasonal-breeding species,
such as the ferret. Ablation of the pituitary in this species during the nonbreeding season causes slight if any change in the testis and accessory reproductive organs. However, when it is removed during the breeding season,
a marked regression to a condition similar to that present during the nonbreeding season occurs (Hill and Parkes, ’33).
 
The experimental result of hypophysectomy on many animal species thus
points directly to this structure as the site of hormonal secretion, particularly
to the anterior lobe (Smith, ’39). The initial work on the relation of pituitary
hormones and the gonad was done upon the female animal. The results of
these studies aroused the question whether one or two hormones were responsible. The latter alternative was suggested by the work of Aschheim and
Zondek (’27) and Zondek (’30) who concluded that two separate substances
appeared to be concerned with the control of ovarian changes.
 
Nevertheless, for a time the concept of only one gonad -con trolling (gonadotrophic) hormone was produced by the pituitary, continued to gain attention, and some workers suggested that the two ovarian elfects of follicular
growth and luteinization of the follicle were due to the length of time of
administration of one hormone and not to two separate substances. However, this position soon was made untenable by research upon the gonadotrophic substances derived from the pituitary gland. Studies along this line
by Fevold, Hisaw, and Leonard (’31) and Fevold and Hisaw (’34) reported
the fractionation, from pituitary gland sources, of two gonadotrophic substances, a follicle-stimulating factor or FSH and a luteinization factor or LH.
This work has been extensively confirmed. It should be observed in passing
that the male pituitary gland contains large amounts of FSH, although, as
mentioned below, the function of the testis and the male reproductive system
relies to a great extent upon the luteinizing factor. Some investigators refer
to the LH factor as the interstitial-cell-stimulating hormone, ICSH. (See Evans,
’47; and also Evans and Simpson in Pincus and Thimann, ’50.)
 
The action of these two hormones upon testicular tissue, according to
present information, is somewhat as follows: If pure follicle-stimulating hormone, FSH, which produces only FSH effects in the female, is injected in
low doses into hypophysectomized male rats, the seminiferous tubules are
stimulated and spermatogenesis occurs. Under these conditions, the interstitial
tissue remains unstimulated and the accessories continue in an atrophic state.
It has further been demonstrated that slight amounts of the luteinizing gonadotrophic hormone, LH (ICSH), added to the above injections of FSH,
effects a much better stimulation of the spermatogonial tissue, and the interstitial tissue also develops well.
 
On the other hand, when pure LH (ICSH) is given alone in small doses,
spermatogenesis is stimulated with slight or no effect upon the male accessory
structures. However, when larger doses of the LH (ICSH) factor alone are
injected, the interstitial tissue is greatly stimulated, and the testicular weight
increases much more than when FSH alone is given. Furthermore, the accessory reproductive structures are stimulated and become well developed, suggesting the elaboration of the male sex hormone. In agreement with these
results, the administration alone of testosterone, the male sex hormone, increases the weight and development of the accessory structures in hypophysectomized animals and it also maintains spermatogenesis. It appears, therefore, that the effects of the LH substance upon the seminiferous tubules and
the accessory organs occur by means of its ability to arouse the formation of
the male sex hormone.
 
A summary of the actions of the pituitary gonadotrophic hormones upon
testicular tissue may be stated as follows:
 
( 1 ) Pure FSH in small doses stimulates the seminiferous tubules and
spermatogenesis with little or no effect upon the interstitial tissue or
the accessory reproductive structures, such as the seminal vesicles or
prostate gland;
 
(2) Small doses of pure LH also stimulate spermatogenesis with little
or no stimulation of the accessory structures;
 
(3) Pure LH (ICSH) in larger doses stimulates the development of the
interstitial tissue with the subsequent secretion of the male sex hormone and hypertrophy of the accessory reproductive organs;
 
(4) The male sex hormone in some way aids or stimulates the process
of spermatogenesis, suggesting that the action of LH occurs through
the medium of the sex hormone (fig. 22).
 
(Consult Evans and Simpson in Pincus and Thimann, ’50, for data and
references; also Turner, C. D., ’48.)
 
The foregoing results of the action of the FSH and LH upon testicular
function might suggest that the LH substance alone is essential in the male
animal. However, it should be observed that without the presence of FSH,
LH is not able to maintain the tubules in a strictly normal manner, the
tubules showing a diminution of size. Also, in extreme atrophic conditions
of the tubules, pure FSH stimulates spermatogenesis better than similar quantities of LH. It is probable that FSH and LH (ICSH) work together to effect
complete normality in the male. This combined effect is known as a synergistic effect. It also is of interest that the injection of small doses of testosterone
propionate into the normal male, with the pituitary gland intact, results in
inhibition of the seminiferous tubules, probably due to the suppression of
pituitary secretion by the increased atnount of the male sex hormone in the
blood. However, high doses, while they likewise inhibit the pituitary, result
in a level of androgen which stimulates the seminiferous tubules directly
(Ludwig, ’50).
 
Aside from the above actions upon testicular tissue by the luteinizing hormone (LH;ICSH) certain other functions of this substance should be mentioned (see fig. 22). One of these is the apparent dependence of the Sertoli
cells upon the presence of the interstitial cells (Williams, ’50). Interstitial
tissue behavior and development in turn relies mainly upon LH (ICSH)
(Fevold, ’39; Evans and Simpson in Pincus and Thimann, ’50). As the sperm
are intimately associated with the Sertoli elements during the latter phases
of spermatogenesis in which they transform from the spermatid into the form
of the adult sperm, a very close association and reliance upon the presence
of the luteinizing hormone thus appears to be established in sperm development.
 
A further study of the LH factor is associated with the maintenance of
the seminiferous tubules themselves. In aged males, the interstitial tissue and
the seminiferous tubules normally involute and regress with accumulation
of large amounts of connective tissue material. In testicular grafts made into
the rabbit’s ear, Williams (’50) found, when interstitial tissue was present
in the grafts, the seminiferous tubules were more nearly normal; when absent,
the tubules underwent fibrosis.
 
Another function of the LH substance apparently is concerned with release
of the sperm from the Sertoli cells. De Robertis, et al. (’46), showed that
anterior pituitary hormones possibly cause release of sperm from the Sertoli
cells in the toad by the production of vacuoles and apical destruction of
the cytoplasm of the Sertoli elements. In testicular grafts Williams (’50) accumulated evidence which suggests that vacuoles and secretion droplets in
the Sertoli cells occurred as a result of LH administration. The combined
results of these investigators suggest that sperm release from the Sertoli cell
is dependent, in some way, upon LH (ICSH) activity.
 
A final function is concerned with the physiological maturing of sperm
in the reproductive duct, at least in many vertebrate species. The well-being
of the epididymis and vas deferens is dependent upon the presence of the
male sex hormone (Creep, Fevold, and Hisaw, ’36). As the male sex hormone results from stimulation of the interstitial cells by the interstitial-cellstimulating substance, LH (ICSH), the connection between this substance
and the physiological maturation of the sperm cell is obvious.
 
2. External Environmental Factors and Testis Function
 
As we have seen above, the anterior lobe of the hypophysis acts as the
main internal environmental factor controlling the testes and, through them,
the reproductive ducts. It has been observed also that food, vitamins, and
anatomical position of the testis are important influences in regulating testicular function. Furthermore, general physiological conditions such as health
or disease have an important bearing upon the gonads (Mills, ’19). All of the above conditions are contained within the body of the organism, and as
such represent organismal conditions.
 
 
Fio. 22. Chart showing the effects of the hypophyseal anterior lobe upon the developing gametes. It also suggests the various factors influencing pituitary secretion of the
gonadotrophic hormones, FSH and LH. Observe that the primitive gamete in the cortex
of the ovary is subjected to the cortical environment and develops into an oocyte, whereas
in the medullary or testicular environment it develops into a spermatocyte. Experiments
upon sex reversal have demonstrated that the medullary and cortical portions of the
gonad determine the fate of the germ cell. In the male area or medulla, the germ cell
differentiates in the male direction, while in the cortex, the differentiation is in the
direction of the female gamete or oocyte, regardless of the innate sex-chromosome constitution of the primitive germ cell. The fate of the germ cell thus is influenced by four
main sets of factors: (1) Internal and external environmental factors, controlling the
secretions of the pituitary body, (2) Fnvironment of the testicular tissue (medulla) and
possible humoral substances produced in this tissue, (3) Environment of the ovarian
tissue (cortex) and possible humoral substances elaborated there, and (4) Secretions of
the anterior lobe of the pituitary body.
 
 
 
 
The following question naturally arises: Do factors or conditions external
to the body impinge themselves in such a way as to control pituitary and
gonadal function?
 
 
a. Light as a Factor
 
Aside from the supply of nutritive substances or the collision of the many
nervous stimuli with the individual which may arouse or depress the sexual
activities, two of the most important obvious external factors are temperature
and light. Research on the reproductive behavior of many animal species,
during the past twenty years, has shown that both of these factors have great
significance on the reproductive activities of many vertebrate species. Bissonnette (’30, ’32, ’35, a and b) has accumulated evidence which demonstrates
that light is a potent factor in controlling the reproductive behavior of the
European starling (Sturnus vulgaris) and also of the ferret (Putorius vulgaris).
In the starling, for example, the evidence shows that green wave lengths of
the spectrum inhibit testicular activity, while red rays and white light arouse
the reproductive function (fig. 23). The addition of electric lighting to each
day’s duration produced a total testis size in midwinter which surpassed the
normal condition in the spring. In the ferret artificially increased day length
beginning at the first part of October brings the testis to maximum size and
activity coupled with a normal mating impulse as early as November and
December (fig. 24). Under normal conditions the male ferret is able to breed
only during February and early March,
 
These findings relative to the influence of light on the reproductive periodicity of animals confirm a fact which has been known for a long time,
namely, that seasonal breeders brought from the northern hemisphere to the
southern hemisphere reverse their breeding season. For example, ferrets which
normally breed from spring to summer in the northern hemisphere shift their
breeding habits to the September-February period when moved to the southern
hemisphere. Inasmuch as the hypophysis is instrumental in bringing about
secretion of the gonadotrophic hormones responsible for the testicular activity,
it is highly probable that light coming through the eyes (see Hill and Parkes,
’33) influences the nervous system in some way arousing the hypophysis and
stimulating it to secrete these substances in greater quantity. However, one
must keep in mind the caution given by Bissonnette, that light is not the only
factor conditioning the sexual cycles of ferrets and starlings.
 
While numerous animals, such as the migratory birds, ferret, mare, many
fish, frogs, etc., normally are brought into a breeding condition during the
period of light ascendency, a large number of animals experience a sexual
resurgence only during the time of year when the light of day is regressing
in span. This condition is found in some sheep, goats, buffalo in nature.
 
 
 
 
 
 
 
 
 
Fig. 23. Sections of testis of the starling (Sturnus vulgaris), showing the effect of
electric lighting added to the bird’s normal daily duration of light during the autumn.
(After Bissonnette, Physiol. Zool., 4.) (A) Inside young control bird — no light added
 
— kept inside as control for (B) from November 9 to December 13. (B) Inside young
 
experimental bird, receiving additional light from “25 watt” bulb from November 9 to
December 13. Total treatment, 34 davs.
 
 
 
 
Fig. 24. Sections of testis and epididymis, showing modification of sexual cycle in the
ferret, Putorius vulgaris, by exposure to increasing periods of light. (After Bissonnette,
’35b.) (A) Seminiferous tubules from normal male over 1 year old, made on October
 
3, no lighting. (B) Epididymis of normal male on October 3, no lighting. (C) Seminiferous tubules of experimental male on November 7, 36 days of added lighting. (D)
Epididymis of experimental males on Nov Tiber 7, 36 days of added lighting.
 
 
deer, some fish, etc. Bissonnette (’41) working with goats found that: “Increasing daily light periods from January 25 to April 5 — followed by diminishing periods until July 5, while temperatures remained normal for the seasons,
with four Toggenburg female goats and one male Toggenburg and one Nubian
female — led to cessation of breeding cycles in February instead of March,
followed by initiation of breeding cycles in May and June instead of September.” In the ewe, Yeates (’47) also found that a change from increasing
daylight to decreasing length of day induced reproductive activity. In a similar
manner. Hoover and Hubbard (’37) were able to modify the sexual cycle
in a variety of brook trout which normally breeds in December to a breeding
season in August.
 
 
b. Temperature Influences
 
In the case of the animals mentioned above, temperature does not appear
to be a major factor in inducing reproductive activity. However, in many
animals temperature is vitally influential in this respect. For example, in the
thirteen-lined spermophile (ground squirrel) Wells (’35) observed that breeding males kept at 40° F. continued in a breeding condition throughout the
year. Under normal conditions this rodent hibernates during the winter months
and comes forth in the spring ready to breed; sperm proliferation and general
reproductive development take place during the period of hibernation. As
the temperature rises during the spring and summer, testicular atrophy ensues,
followed by a period of spermatogenesis and reproductive activity when the
lowered temperatures of autumn and winter come again. Light, seemingly, is
not a factor in this sexual cycle. Another instance of temperature control
occurs in the sexual phase of the common red newt, Triturus viriciescens. Here
it is the rising temperature of the summer which acts as the inducing agent,
and sperm thus produced are discharged into the accessory ducts during the
fall and winter to be used when copulation occurs in early spring. However,
if this species is kept at a relatively low temperature of 8 to 12° C. during
the summer months, spermatogenesis is inhibited and the testis regresses. In
the stickleback, Gasterosteus aculeatus, as reported by Craig-Bennett (’31),
spermatogenesis occurs during July to early September and appears to be
conditioned by a rising temperature, whereas the interstitial tissue and the
appearance of secondary sexual features reach their greatest development
under increased light conditions and slowly rising temperatures (fig. 15).
Bissonnette, in his work on ferrets, also observed a difference in the behavior
of these two testicular components; the interstitial tissue responds to large
increases of daily light periods, whereas the seminiferous tubules are stimulated by small, gradually increasing periods of light.
 
The above examples emphasize the importance of a single environmental
factor on the pituitary-gonadal relationship. However, in the hedgehog,
Allanson and Deansley (’34) emphasize temperature, lighting, and hormone
injections as factors modifying the sexual cycles, while Baker and Ransom
(’32, ’33, a and b) show that light, food, temperature, and locality affect
the sexual cycles and breeding habits of the field mouse. In some vertebrates,
therefore, a single factor may be the dominant one, whereas in others, numerous factors control the action of the pituitary and reproductive system.
 
E. Internal Factors Which May Control Seasonal and Continuous Types
of Testicular Function
 
In endeavoring to explain the differences in response to external environmental factors on the part of seasonal and continuous breeders, one must
keep in mind the following possibilities:
 
(1) The anterior lobe of the hypophysis in some forms (e.g., ferret)
cannot be maintained in a secretory condition after it has reached its
climax; that is, it apparently becomes insensitive to the light factor. As a
result, regression of the pituitary and testis occurs (Bissonnette, ’35b).
 
(2) In the starling, the anterior hypophysis may be maintained by the
lighting, but the testis itself does not respond to the presence of the
hypophyseal hormones in the blood (Bissonnette, ’35b). The possibility in this instance may be that testicular function wanes because
the body rapidly eliminates the hormone in some way (see Bachman,
Collip, and Selye, ’34).
 
(3) Consideration also must be given to the suggestion that the activities
of the sex gland by the secretion of the sex hormone may suppress
anterior lobe activity (Moore and Price, ’32).
 
We may consider two further possibilities relative to continuous testicular
function :
 
(4) If the “brake actions” mentioned above are not present or present
only in a slight degree, a degree not sufficient to interrupt the activities
of the anterior lobe or of the sex gland, a more or less continuous
function of the testis may be maintained.
 
(5) When several or many environmental factors are concerned in producing testicular activity, a slight altering of one factor, such as light,
may prove insufficient to interrupt the pituitary-germ-gland relationship, and a continuous breeding state is effected in spite of seasonal
changes.
 
Underlying the above possibilities which may control testicular function is
the inherent tendency or hereditary constitution of the animal. In the final
analysis, it is this constitution which responds to environmental stimuli, and
moreover, controls the entire metabolism of the body. In other words, the
above-mentioned possibilities tend to oversimplify the problem. The organism as a whole must be considered; reproduction is not merely an environmentalpituitary-sex gland relationship.
 
F. Characteristics of the Male Reproductive Cycle and Its Relation to  
Reproductive Conditions in the Female
 
As indicated above, reproduction in the male vertebrate is either a continuous process throughout the reproductive life of the individual or it is a
discontinuous, periodic affair. In the continuous form of reproduction the
activities of the seminiferous tubules and the interstitial or hormone-producing
tissues of the testis function side by side in a continuous fashion. In the
discontinuous, periodic type of testicular function, the activities of the seminiferous tubules and of the interstitial tissue do not always coincide. The
activities of the seminiferous tubules, resulting in the production of sperm
for a particular reproductive cycle, tend to precede, in some species by many
months, the activities of the sex-hormone-producing tissue. Evidently, the
output of the FSH and LH substances from the pituitary gland are spread
out over different periods of the year to harmonize with this activity of the
testicular components.
 
It will be seen in the next chapter that a continuous breeding faculty is
not present in the female comparable to that of the male. All females are
discontinuous breeders. In some species, the cycles follow each other with
little rest between each cycle unless the female becomes pregnant or “broody.”
Some have a series of cycles over one part of the year but experience sexual
quiescence over the remaining portion of the year. However, in most female
vertebrates there is but one reproductive cycle per year.
 
In harmony with the above conditions, the continuous variety of testicular
function is always associated with the condition in the female where more
than one reproductive cycle occurs per year. Continuous reproductive conditions in the male, therefore, are adapted to serve one female two or more
times per year or several different females at intervals through the year.
Furthermore, the complicated, highly glandular, greatly extended type of
male-reproductive-duct system is adapted to conditions of (1) continuous
breeding, or (2) service to more than one female during one breeding season
of the year, whereas the simple type of reproductive duct is adapted to the
type of service where all or most of the genital products are discharged during
one brief period. In other words, the entire male reproductive system and reproductive habits are adapted to the behavior of female reproductive activities.
 
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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

   Comparative Vertebrate Embryology 1953: 1. The Period of Preparation | 2. The Period of Fertilization | 3. The Development of Primitive Embryonic Form | 4. Histogenesis and Morphogenesis of the Organ Systems | 5. The Care of the Developing Embryo | Figures
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Part I The Period of Preparation

Part I - The Period of Preparation: 1. The Testis and Its Relation to Reproduction | 2. The Vertebrate Ovary and Its Relation to Reproduction | 3. The Development of the Gametes or Sex Cells

The events which precede the initiation of the new individual's development are:

  1. The preparation of the male and female parents and their reproductive structures for the act of reprcxluction (Chaps. 1 and 2).
  2. The preparation of the gametes (Chap. 3).

The anterior lobe of the pituitary gland, because of its secretion of the gonadotrophic (gonad-stimulating) hormones, is the pivotal structure in the reproductive mechanism.

The gonadotrophic hormones are:

  1. Follicle-stimulating hormone, FSH;
  2. Luteinizing hormone, LH (ICSH), and
  3. Luteotrophin, LTH.

The Testis and Its Relation to Reproduction

1. The Testis and Its Relation to Reproduction

A. Introduction

  1. General description of the male reproductive system
  2. Importance of the testis


B. Anatomical features of the male reproductive system

  1. Anatomical location of the testis
  2. Possible factors involved in testis descent
  3. General structure of the scrotum and the testis in mammals a. Structure of the scrotum b. General structure of the testis
  4. Specific structures of the mammalian testis which produce the reproductive cells and the male sex hormone a. Seminiferous tubules b. Interstitial tissue
  5. The testis of vertebrates in general
  6. Accessory reproductive structures of the male a. The reproductive duct in forms utilizing external fertilization b. The reproductive duct in species practicing internal fertilization c. Specific activities of the various parts of the male reproductive system

The Vertetrate Ovary and Its Relationship to Reproduction

2. The Vertebrate Ovary and Its Relation to Reproduction

A. The ovary and its importance

B. Preformationism, past and present

C. General structure of the reproductive system of the vertebrate female

1. General structure of the ovary

2. General structure of the accessory reproductive organs

D. Dependency of the female reproductive system on general body conditions

1. Inanition

2. Vitamins

a. Vitamin A

b. Vitamin B

c. Vitamin C

d. Vitamin E

3. The hypophysis (pituitary gland)

E. Activities of the ovary in producing the reproductive state

1. The ovary as a “storehouse” of oogonia

2. Position occupied by the primitive female germ cells in the ovarian cortex

3. Primary, secondary, and tertiary follicles of de Graaf

4. Hormonal factors concerned with the development of egg follicles

a. Effects produced by the gonadotrophic hormones of the development of the mammalian egg follicle

b. Stimulating effects of the gonadotrophins on the ovaries of other vertebrates

5. Structure of the vertebrate, mature egg follicle

a. Structure of the mature follicle in metatherian and eutherian mammals

b. Structure of the prototherian egg follicle

c. Egg follicles of other vertebrates

6. Ovulatory process; possible factors controlling ovulation

a. Process of ovulation in higher mammals

1) Changing tissue conditions culminating in egg discharge from the ovary

2) Hormonal control of the ovulatory process

b. Ovulation in vertebrate groups other than the higher mammals

1) Hen

2) Frog

3) Hormonal control of ovulation in lower vertebrates

c. Comparison of the immediate factors affecting egg discharge in the vertebrate group

7. Internal conditions of the ovary as an ovulatory factor

8. Number of eggs produced by different vertebrate ovaries

9. Spontaneous and dependent ovulation in the mammals and in other vertebrates

10. Egg viability after discharge from the ovary

11. History of the egg follicle after ovulation

a. Follicles which do not develop a post-ovulatory body

b. Follicles which develop a post-ovulatory body; formation of the corpus luteum

12. Hormones of the ovary and their activities in effecting the reproductive condition

a. Estrogenic hormone

1) Definition and source of production

2) The ovary as the normal source of estrogen in the non-pregnant female

3) Pituitary control of estrogen formation

4) Effect of estrogen upon the female mammal

5) Effects of estrogen in other vertebrates

b. Progesterone — the hormone of the corpus luteum

1) Production of progesterone

2) Effects of progesterone

F. Reproductive state and its relation to the reproductive cycles in female vertebrates

1. Sexual cycle in the female mammal

a. Characteristics and phases of the reproductive cycle

b. Relation of cstrus and ovulation in some common mammals

1) Spontaneously ovulating forms (Sexual receptivity of male occurs at or near time of ovulation)

2) Dependent ovulatory forms (Sexual receptivity [heat] occurs previous to time of ovulation)

c. Non-ovulatory (anovulatory) sexual cycles

d. Control of the estrous cycle in the female mammal

e. Reproductive cycle in lower vertebrate females

G. Role of the ovary in gestation (pregnancy)

1. Control of implantation and the maintenance of pregnancy in mammals

2. Gestation periods, in days, of some common mammals

3. Maintenance of pregnancy in reptiles and other vertebrates

H. Role of the ovary in parturition or birth of the young

I. Importance of the ovary in mammary-gland development and lactation

J. Other possible developmental functions produced by the ovary

K. Determinative tests for pregnancy

The Development of the Gametes or Sex Cells

3. The Development of the Gametes or Sex Cells

A. General considerations

B. Controversy regarding germ-cell origin

C. Maturation (differentiation) of the gametes

1. General considerations

2. Basic structure of the definitive sex cell as it starts to mature or differentiate into the male meiocyte (i.e., the spermatocyte) or the female meiocyte (i.e., the oocyte)

3. Nuclear maturation of the gametes

a. General description of chromatin behavior during somatic and meiotic mitoses

b. Reductional and equational meiotic divisions and the phenomenon of crossing over

c. Stages of chromatin behavior during the meiotic prophase in greater detail

1) Leptotene (leptonema) stage

2) Zygotene or synaptene (zygonema) stage

3) Pachytene (pachynema) stage

4) Diplotene (diplonema) stage

5) Diakinesis

d. Peculiarities of nuclear behavior in the oocyte during meiosis; the germinal vesicle

e. Character of the meiotic (maturation) divisions in the spermatocyte compared with those of the oocyte

1) Dependent nature of the maturation divisions in the female meiocyte

2) Inequality of cytoplasmic division in the oocyte

f. Resume of the significance of the meiotic phenomena

4. Cytosomal (Cytoplasmic) maturation of the gametes

a. General aspects of the cytoplasmic maturation of the gametes

b. Morphogenesis (spermiogenesis) ( sperm ioteleosis) of the sperm

1) Types of sperm

2) Structure of a flagellate sperm

a) Head

b) Neck

c) Connecting body or middle piece

d) Flagellum

3) Spermiogenesis or the differentiation of the spermatid into the morphologically differentiated sperm

a) Golgi substance and acroblast; formation of the acrosome

b) Formation of the post-nuclear cap

c) Formation of the proximal and distal centrioles; axial filament

d) Mitochondrial material and formation of the middle piece of the sperm

e) The cytoplasm, axial filament, mitochondria, and tail formation


c. Cytoplasmic differentiation of the egg

1) Types of chordate eggs

a) Homolecithal (isolecithal) eggs

b) Telolecithal eggs

2) Formation of the deutoplasm

3) Invisible morphogenetic organization within the cytoplasm of the egg

4) Polarity of the egg and its relation to body organization and bilateral symmetry of the mature egg

5) Membranes developed in relation to the oocyte; their possible sources of origin

a) Chorion in St ye la

b) Egg membranes of Amphioxus

c) Vitelline membrane and zona radiata of elasmobranch fishes

d) Zona radiata of teleost fishes

e) Vitelline membrane (zona radiata) in amphibia

f) Zona radiata (zona pellucida) of the reptile oocyte

g) Vitelline membrane (zona radiata) of the hen’s egg

h) Membranes of the mammalian oocyte

5. Physiological maturation of the gametes

a. Physiological differentiation of the sperm

b. Physiological ripening of the female gamete

D. Summary of egg and sperm development

Part I - The Period of Preparation: 1. The Testis and Its Relation to Reproduction | 2. The Vertebrate Ovary and Its Relation to Reproduction | 3. The Development of the Gametes or Sex Cells

Cite this page: Hill, M.A. (2024, June 26) Embryology Book - Comparative Embryology of the Vertebrates 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Comparative_Embryology_of_the_Vertebrates_1

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
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